1. Field of the Invention
This invention relates to a color plasma display panel which is used for an information displaying terminal or a flat type television set, and particularly to a panel structure to achieve a display with high contrast and brightness.
2. Prior Art
A color plasma display panel is a display device which excites phosphors to cause them to emit visible light by ultra-violet rays generated as a result of gas discharge, thereby achieving a color display. The color plasma display panel can be divided into AC and DC types according to the formation of gas discharge. Of the two, AC type is better than DC type in brightness, luminous efficiency, and life. Of AC type variations, a reflection AC type is excellent in brightness and luminous efficiency.
FIG. 1 shows the cross-section of one example of a conventional reflection AC type color plasma display. On a transparent glass substrate 1 is formed a discharge electrode 2 made of a transparent material. A plurality of discharge electrodes are disposed like belts in a direction parallel to the surface of the drawing. Between given adjacent discharge electrodes 2 is applied an AC voltage in the form of a series of pulses with a frequency of several tens kHz to several hundreds kHz to obtain a discharge for display.
With the reflection type AC surface discharge color plasma display, a transparent electrode made of tin oxide (SnO.sub.2) or indium tin oxide (ITO) has been used as the discharge electrode 2 so that visible light from phosphors may not be intercepted. However, the foregoing transparent material has a comparatively high sheet resistance. Therefore, when incorporated into a large-sized panel or a high precision panel, it gives an electric resistance as high as several k.OMEGA., and even when a voltage is applied thereto, the voltage does not rise sufficiently rapidly to achieve a smooth driving. As a measure to solve this problem, a thin metal layer of either a laminated thin film composed of chromium/copper/chromium or an aluminum thin film, or a thick metal layer such as silver is provided on certain parts of the transparent electrode to form a bus electrode (not shown), thereby enabling a discharge electrode to have a lowered electric resistance.
On this discharge electrode 2 are formed color filter layers 3r, 3g and 3b each made of a pigment particle layer extending in the form of a stripe as if to intersect at right angles with the discharge electrode 2. Generally, for the color filter layer 3 is used a material which has an optical property to transmit only the optimum element of emitted light from a phosphor layer 8 placed opposite thereto. Further, the color filter layers 3 are covered with a transparent dielectric layer 4. This dielectric layer 4 has a function to limit an electric current characteristically associated with an AC type plasma display. The dielectric layer 4 is usually produced after a paste mainly composed of lead glass with a low melting point has been coated, and fired at a temperature higher than the softening point thereof, and the glass been allowed to flow on the surface to form a smooth layer having a thickness of 20-40 .mu.m with no air bubbles captured therein, because this method allows an easy production of the layer in question and will impart it a property to keep insulation in the presence of a higher voltage.
Next, a protecting layer (not shown) is formed to cover the whole expanse of dielectric layer 4. This is a thin MgO layer formed by chemical vapor deposition process or by sputtering process, or a thick layer of MgO formed by printing process or by spraying process. It has a thickness of about 0.5-1 .mu.m. The role of this protecting layer is a reduction of a discharge voltage and a prevention of a surface sputtering.
On the other hand, on a back substrate 5 are formed address electrodes 6 upon which display data pulses are applied. In FIG. 1, address electrodes 6 extend in a direction normal to the surface of the drawing, and are formed on places corresponding with red, green and blue phosphor layers 8 arranged in the form of a stripe as will be described later. Namely, the address electrode 6 intersects the discharge electrode 2 on the front substrate 1 with right angles. These address electrodes 6 are covered with a white dielectric layer 7 which is produced after lead glass with a low melting point and a white pigment has been mixed to produce a paste for a thick layer which has then been printed and baked. The white pigment usually includes a titanium oxide powder or aluminum oxide powder. On this white dielectric layer 7 are formed barriers 9 to define spaces for electric discharge, usually by printing process. Further, on the top of barrier is thickly printed a paste usually consisting of a metal oxide powder of iron, chromium or nickel, and glass with a low melting point, and thus the top, having a color of black, is prevented from reflecting light coming from a bright environment. Furthermore, the barriers 9 are also effective in preventing wrongly induced electric discharges or optical cross-talks which would otherwise take place between adjacent discharge cells.
On the surface of discharge cell 10, phosphors 8r, 8g and 8b which give red, green and blue visible lights, respectively are coated one time for each phosphor, or three times for all the phosphors. Each phosphor is coated on the side walls of barrier, to increase the phosphor coat area, thereby achieving a higher brightness. Coating of the phosphor usually takes place by screen printing.
Later, the front substrate 1 and back substrate 5 are placed opposite so that the discharge electrodes 2 of the former and the address electrodes of the latter intersect with each other with right angles with the barriers in between, and their joints are hermetically sealed air-tight. A dischargeable gas, for example, a mixture of He, Ne and Xe is injected into the interior of discharge cells under a pressure of about 500 Torr.
In FIG. 1, two discharge electrodes are assigned to each discharge cell, and a surface discharge is generated in the gap between these discharge electrodes to produce a plasma in the discharge cell. Vacuum ultra-violet rays generated during this process excite red, green and blue phosphors 8r, 8g and 8b, and cause them to emit visible light which are filtered through filters 3 on the front substrate 1 to give light for display.
One of a pair of adjacent discharge electrodes acts as a scan electrode and the other acts as a sustain electrode. To continue the discharge, sustain pulses are applied between the scan electrode and sustain electrode. To generate a write discharge, a voltage is applied between the scan electrode and address electrode 6 to allow an across-discharge to be generated therebetween, which is then taken over by sustain pulses subsequently imparted to develop into a sustaining discharge between the surface discharge electrodes.
A phosphor used in a color plasma display panel is composed of a white powder having a high reflective index. With the conventional color plasma display panel described above, when external light from indoors as well as outdoors impinges on the panel, the majority of light is absorbed by the top of barriers and bus electrodes, but about 30-50% thereof is reflected back, thereby greatly impairing the contrast and color purity of display. To avoid such reflection of external light and ensure a high contrast display, a method may be employed which consists of the use of a neutral density (ND) filter with a light transmission of 40-80%, but as that filter will cut some of visible elements of the luminescence from phosphor, the luminance of display on the panel will be reduced.
As a method whereby it is possible to minimize the reflection of external light without impairing the luminance of display as much as possible, introduction of color filters 4 has been proposed. This method consists of placing color filters 4 allowing red, green and blue light to pass on the viewing side in correspondence with red, green and blue luminescence emitted from respective discharge cells.
To introduce such color filters into an AC type plasma display, there have been known two methods: one is to place the filters directly on the surface of glass substrate, and the other is to prepare a dielectric layer necessary for the AC type plasma display as a colored glass layer.
A conventional example of a color plasma display panel incorporating such color filters can be seen, for example, in FIG. 6 of Japanese Unexamined Patent Publication No. 6-5202.
The conventional color filters are produced after materials usually containing a pigment powder as a main ingredient corresponding to respective colors have been prepared, applied to a substrate to form layers thereupon separately for each chromatic component, and fired. Incidentally, as the pigment powder must withstand a high temperature (500-600.degree. C.) during firing process, inorganic materials must be chosen therefor. Representative pigment powders are shown below.
Red: Fe.sub.2 O.sub.3 compounds PA1 Green: CoO--Al.sub.2 O.sub.3 --Cr.sub.2 O.sub.3 compounds PA1 Blue: CoO--Al.sub.2 O.sub.3 compounds
As the aforementioned filter layers are printed separately for the three color components, red, green and blue, namely, printing takes place three times in total to complete the formation of entire color filter layers, seams, grooves or steps may be generated between adjacent color filters. These flaws may damage insulations between other circuit elements or have adverse effects on the later processes necessary for the formation of black barriers.
To avoid such adverse effects as described above, a method is presented whereby the color filter made of colored glass with a low melting point is further coated by a transparent dielectric layer thereby to smoothen the surface of color filter. The structure of such color filter is disclosed in Japanese Unexamined Patent Publication No. 7-01924. Alternatively, different pigments corresponding to those primary colors have been pasted separately, and then glass paste with a low melting point is printed on the entire surface, and the assembly is fired, to allow thereby the pigments to disperse or diffuse into the glass layer (Japanese Unexamined Patent Publication No. 4-245140 is referred to).
As these conventional color plasma display panels have color filters corresponding to respective visible components of luminescence placed on a substrate on the side of display, and thus is capable of suppressing the reflection of external light in the manner as described above, it can achieve a high contrast display. As the color filter is so transmissible to visible rays as to pass about 60-80% of rays having a mid wavelength of the spectra corresponding to red, green or blue light, 40-20% loss in brightness results. To achieve the same degree of contrast with above, the ND filter must have a transmission of about 50% to visible rays. Accordingly, as long as contrast being kept the same, a color filter can allow more light to pass than does the ND filter, or a color filter can allow a higher luminance than does the ND filter. Furthermore, luminescence from the phosphor can be modified through adjustment of the properties of color filter so as to optimize the color purity and hue of the luminescence. In addition, the color filters can intercept visible rays (for example, orange rays from Ne gas) emitted from a discharge gas, and thus contributes to a widening of reproducible color range.
On the other hand, when a high AC voltage pulse is applied to generate a discharge for display, an impulse current is generated, and flows through a driving circuit and color plasma display panel. This impulse current causes an electromagnetic field to radiate. A method to suppress the radiation of electromagnetic field from the display surface consists of placing an electroconductive electromagnetic field shielding plate on the front surface of display, and connecting its margins to the housing to ground it. What is disclosed in Japanese Unexamined Patent Publication No. 4-134900 is generally known as a conventional example incorporating such structure. The electromagnetic field shielding plate generally consists of a transparent insulating plate of an acryl resin or glass with a flat thin film applied thereupon as an electrode, or the same plate with a mesh made of electroconductive fibers bonded thereupon. In the conventional example described above, an indium tin oxide (ITO) film is used. The surface resistance of this electromagnetic field shielding film is preferably 1 .OMEGA./.quadrature. or less, but generally a thin transparent film electrode highly transmissible (about 80%) to visible light generally has a surface resistance of 10 .OMEGA./.quadrature. or less, and thus its electromagnetic field shielding effect is insufficient. Thus, various trials have been made to reduce the surface resistance of transparent electroconductive film without sacrificing its transmission to visible light, for example, by adjusting the conditions of film formation or by applying various metal films thereupon.
By contrast, a mesh of electroconductive fibers gives a surface resistance of 0.1 .OMEGA./.quadrature., and has a sufficient electromagnetic field shielding activity. However, when the mesh is applied on the surface of display, it generates a moire pattern through interference with a frame pattern of display cells. This moire pattern can be made negligible through adjustment of the diameter of wires constituting the mesh, size of mesh opening, and angle of mesh with respect to the display surface, but can not be annihilated. Further, the wires constituting the mesh narrows the view angle. Furthermore, as the mesh is produced after resin fibers have been woven into a cloth upon which a metal such as copper or nickel has been plated, its mesh opening has a limitation in size, and passes only about 50-60% of visible light incident on the mesh.
The color plasma display panel dependent on the use of color filters also requires the placement of an electromagnetic field shielding plate as an essential element for the reasons as described above. Thus, when a mesh of electroconductive fibers having a low transmission to visible light is introduced to shield electromagnetic fields in the manner as described above, it cancels out the high contrast effect brought about by the introduction of color filters, and damages advantages and effects imparted to the color plasma display panel incorporating color filters, and thus disables the practical application thereof.
The conventional color plasma display panel with color filters, although being capable of providing a display high in contrast and brightness, and wide in color reproducibility as described above, requires the use of an electromagnetic field shielding plate to shield electromagnetic fields as an essential element, but, as the plate in question having a low transmission to visible light, its existence lowers the brightness of display, and cancels out the advantages brought about by the color filters. For this reason, it has been difficult to put the color plasma display panel incorporating color filters into practice.